fix(p2p): add exponential backoff for reacher

[gacevicljubisa]

Checklist

• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
• I have filled out the description and linked the related issues.

Description

• Implements exponential backoff for peer ping retries, with separate caps for success and failure:
  • Success: 10m -> 20m (capped at 2^2 x base)
  • Failure: 10m -> 20m -> 40m -> 80m (capped at 2^4 x base)
• Adds configurable jitter (±20%) to retry intervals to prevent synchronized retry storms.
• Reduces the number of worker goroutines from 16 to 8.
• Tracks consecutive ping successes and failures per peer to adjust backoff dynamically.
• Resets backoff counters on reconnect and uses a generation counter to discard stale ping results from in-flight pings that were dispatched before the reconnect.
• Uses a far-future sentinel (time.Hour) for in-flight peers instead of the base retry duration, preventing re-dispatch while a ping is active.
• Adds Prometheus metrics for the reacher: queue depth (bee_reacher_peers), ping attempt/error counters, and a ping duration histogram with buckets tuned for the 15s ping timeout.
• Updates and expands unit tests to cover backoff, cap, reconnect-reset, and jitter logic. Tests disable jitter (JitterFactor: 0) for deterministic timing under synctest.

Open API Spec Version Changes (if applicable)

Motivation and Context (Optional)

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